Trace Analysis Guide - Container Material Properties

Materials

Laboratories use many different container materials for handling samples during sample preparation. Some materials are more advantageous to use than others. In this part, we'll look at the properties of borosilicate glass, porcelain, quartz, platinum, graphite, and plastics.

Borosilicate Glass

Borosilicate glass is used extensively. It is resistant to most acids, but should not be used with HF or boiling H₃PO₄. As a general rule alkaline solutions should not be heated or stored in borosilicate glass. Borosilicate glass can contribute a variety of contaminants. It should not be heated over temperatures achievable using a hot plate (500 °C). For example, if you need to ash a sample using a muffle furnace, do not use borosilicate glass.

Porcelain

Porcelain is a popular material used for ashing purposes. Porcelain contains Na, K, Al, and Si in increasing concentration. It is typically coated with a glaze which is about 70 % SiO₂, with roughly equal amounts of the oxides of Al and Ca, and lesser amounts of Na and K. Attack will occur if the sample contains even minor amounts of the alkali metals. This is made evident by a dulling in the normally shiny surface. If alkalis are present, then the sample is typically treated with conc. H₂SO₄ prior to ashing. The following should not be heated in porcelain: HF; boiling H₃PO₄; and the oxides, hydroxides, or carbonates of the alkali or alkaline earth elements. The major advantage or porcelain over glass is that it can be heated up to 1100 °C.

Quartz

There are two types of quartz -- opaque and transparent.

Opaque quartz has the highest trace element concentration and should not be used for trace analysis.
Transparent quartz comes in four different varieties;
Types I & II are made from naturally occurring quartz crystals or sands. Type I is created by electric melting and type II by flame melting. Type II has slightly less impurities than type I (some impurities are volatilized by the flame).

Type III quartz is made synthetically by vapor phase hydrolysis of pure silicon compounds such as SiCl₄. This type of quartz is more pure than the natural quartz, with the exception of Cl ^- which is ~ 50 ppm.

Type IV quartz is synthetically made from SiCl₄ using a process involving electrical fusion of the oxidized staring material. It is as pure as type III, with respect to trace metal content, and contains far more Cl^-.

Use the synthetic type III quartz whenever possible. More details as to the contamination issues around the use of quartz will be discussed in later chapters.

Quartz is typically 99.8+% SiO₂. It is attacked by HF, boiling H₃PO₄, and the alkali and alkaline earth oxides, hydroxides, and carbonates. It can be heated to 1100 °C . Its main advantage over that of porcelain is that major contamination occurs from only Si -- however, this contamination can be significant.

Platinum

Platinum, although expensive, is a popular container material. It heats up and cools down rapidly, making it excellent for % ash determinations where the % ash is at low levels.

It is resistant to attack by most acids and reagents. Avoid concentrated H₃PO₄ at high temperatures, HCl + HNO₃ mixtures and fusions using Li₂CO₃, Na₂O₂, or the alkali hydroxides. Fusions using Na₂CO₃ are common in addition to fusions using the alkali borates, fluorides, nitrates, and bisulphates. Avoid heating at prolonged temperatures in excess of 1100 °C (m.p. = 1772 °C).

Platinum can be destroyed by heating with metals with which it can alloy. Avoid high temperature heating with samples containing significant levels of any metal that may be in or reduced to the metallic state during the heating process. For example, a sample containing high levels of Cu⁰ or Cu⁺² should be avoided, especially if present in an organic matrix. A sample containing trace levels of Cu in an organic matrix will not ruin the platinum, but it is likely to be lost to the platinum during the ashing process. Since platinum has this alloying tendency, it is best to avoid its use with samples containing elements other than those that have no tendency to form the metal (i.e. - alkali, alkaline and rare earth elements). Platinum is known to contain trace amounts of the other precious metals and should not be used for their preparation. Avoid samples containing Hg any an form. Hg metal is easily formed and alloys very readily at room temperature with platinum. Also avoid ashing samples containing P in any form, including the phosphates.

Graphite

Graphite is very inexpensive and relatively clean, but very messy to work with. It is an inexpensive way to perform Li₂CO₃ fusions where the crucible slowly oxidizes away over the course of 7-10 fusions. It is popular because it does not wet by some melts which can be poured out quantitatively. Losses due to the porosity of graphite should exclude its use for ashing samples containing trace metals. Graphite's main advantage to the trace analyst is being a material that can withstand fusions that might destroy platinum. Our chemists use graphite for performing Li₂CO₃ fusions in the preparation of large numbers of limestone samples for major minor and trace elemental analysis.

Plastics

Plastics are very important to the trace analyst. Whenever possible, the analyst should attempt to use plastics for sample collection, storage, preparation, and measurement. Their major disadvantage is the inability to be used for high temperature operations, such as ashing or fusion. Table 5.1 shows a summary of the physical properties of some common plastics.

Table 5.1: Physical Properties of Common Plastics

FEP (FLUORINATEDETHYLENEPROPYLENE)
PFA (PERFLUOROALKOXY)
FLEP (FLUORINATED HIGH-DENSITY POLYETHLYENE)
PMP (POLYMETHYLPENTENE)
PP (POLYPROPYLENE)
HDPE (HIGH-DENSITY POLYETHYLENE)
LDPE (LOW-DENSITY POLYETHYLENE)

The most popular plastics are PFA and HDPE / LDPE.

PFA has excellent properties, allowing for use in acid digestions up to 250 °C. Typically, PFA is used for acid digestions using either HF, HNO₃ or HCl, alone or in combination. The use of higher boiling acids such as H₂SO₄ and H₃PO₄ have been reported in PFA, but great care must be taken not to exceed the 250 °C maximum operating temperature. PFA is commonly used in the construction of microwave digestion vessels. Microwave digestions using the higher boiling acids should not be attempted. Digestions using HCLO₄should never be performed in plastics of any kind.

LDPE or HDPE bottles are typically used for containment of the sample digestate after dilution with water. These bottles can withstand solutions of HNO₃ that are 10% v/v and lower over extended periods of time (i.e. - years). The caps used for the LDPE and HDPE bottles are made of PP, which is more rigid than the polyethylene and well-suited for its purpose. Unfortunately, the PP cap is not as clean as the PE bottle.

All of the above plastics were part of a study conducted at our laboratories concerning their purity and cleaning properties, further described below.

The Purity and Cleaning of Plastics

Contamination issues with plastic containers, although less severe than other container types, are critical:

"The determination of trace elements at very low levels, particularly in liquid samples, has been found to be biased by the analytical blank and can often be attributed in large part to contamination from sampling and storage containers."¹

Conventional sample handling methods were compared to clean techniques for individual 35 steps. These steps covered sample collection, storage, preparation and measurement of water samples for Ag, Cd, Cu and Pb. It was reported that two thirds of all steps contributed statistically significant amounts of contamination in the measurement of dissolved and particulate Cd, Cu, and Pb -- the average contamination for a single contributing step was 300% (Cd), 141% (Cu), and 200% (Pb).²

This section includes a closer look at part of a plastic container materials study conducted by Inorganic Ventures' technical staff³. Areas of interest in this study included the following:

Availability and price (see Table 5.1)
Chemical resistance (see Table 5.1)
Physical properties (including transpiration) (see Table 5.1, transpiration data presented in the next part of this series)
Identify (+) contaminants
Determine (+) contaminants level
Effectively remove (+) contaminants
Identify (-) contamination (adsorption)

The plastics chosen for this study are shown above in Table 5.1. Results of this study will be cited to illustrate points throughout this chapter. This section will address the identity, level, and removal of contaminants in these plastics.

- Experimental Design -

All containers were filled and handled in a clean area.
"Conductivity" water and doubly distilled nitric acid were used.
All leaching was performed in an oven at 60 °C.
All measurements were made using an ICP-MS, located in a clean room.
The leachate solution was measured directly from the container under study.
Leaching solutions were exposed only to the container under study.
Leaching solutions were only exposed to ULPA filtered air.

- Experiment Results -

Table 5.2: Element Concentrations in Leaching Solution
(noted in ng/mL after 59 hours at 60 °C)

All of the containers involved in the leaching study were leached a second time. The results were that all of the elements detected were removed during the first leaching. These results, along with the leaching time study, is shown for Fe in Table 5.3.

Table 5.3: Leaching Behaviour of Iron

The above table shows that most of the Fe was leached after 19 hours, with the exception of one of the HDPE bottles (HDPEb). We currently use a leaching time of 72 hours, due to the apparent unpredictability of the necessary leaching time. However, as shown above for Fe (and other elements not shown for the sake of brevity), the 59 hour / 60 °C leaching time / temperature combination was complete. A 66 hour / °C re-leach of these bottles using either 1% HNO₃ or water showed no Fe above the detection limit of 0.02 ng/mL.

The relative purity of the plastics as received from the manufacturer is shown in Figure 5.1 below.

Figure 5.1: Packaging Container Purity
(ranking from leaching study)

Most surprising was the lack of impurities found. This may in part be due to the clean room conditions used to perform both the leaching and the measurement steps. Figure 5.1 can be used in conjunction with Table 5.1 to identify the cleanest plastic having the chemical and physical properties for a given operation. This study, for example, suggests LDPE for sample storage and collection while PFA would be more suitable as a plastic for use in microwave acid digestions.

1. J. R. Moody, R. M. Lindstrom, Analytical Chemistry 49 (1977): p. 2264.
2. G. Benoit, K. Hunter, T. Rozan, "Sources of Trace Metal Contamination Artifacts During Collection, Handling, and Analysis," Analytical Chemistry 69 (1997): 1006-1011.
3. Paul R. Gaines, "Contamination Issues in Trace Elemental Analysis," presented at FACSS, Detroit, MI (October 2001).